fix postData's operationName on http-post example

Author: qustavo

examples/http-post/main.go

@@ -11,7 +11,7 @@ import (
 
 type postData struct {
 	Query     string                 `json:"query"`
-	Operation string                 `json:"operation"`
+	Operation string                 `json:"operationName"`
 	Variables map[string]interface{} `json:"variables"`
 }